1. Field of the Invention
This invention relates to the preparation of polyimides end-capped with anhydride or other groups. More specifically it relates to a process for the preparation of such polyimides by the reaction of aromatic dianhydrides with aromatic diisocyanates. Still more specifically it relates to such reaction conducted in the presence of a metal acetylacetonate such as aluminum acetylacetonate. Still more specifically this process allows the use of lower temperatures and avoids certain destructive effects such as the reduction of the anhydride activity and thereby makes them more effective in a reaction with epoxy compounds.
2. State of the Prior Art
It has been found that when a polyimide having terminal anhydride groups is prepared by the reaction of an aromatic dianhydride with an aromatic diamine with sufficient excess dianhydride to give terminal anhydride groups in the presence of an amide solvent such as dimethylformamide (DMF), dimethylacetamide (DMA), N-methyl-2-pyrrolidone (NMP), etc., the anhydride activity is reduced, apparently by reaction between the anhydride and the amide groups in the solvent. It is believed that this undesirable side reaction is favored by the high temperatures required to complete cyclization or imidization for formation of the polyimide structure.
In the use of polyimide dianhydride-polyepoxide reaction products for use in laminations, coatings, etc. it is desirable to prepare such reaction products dissolved in concentrations of 20% by weight or more. U.S. Pat. No. 3,663,651 describes the reaction of polyepoxide with various polyimide dianhydrides. Throughout the teaching and working examples of this patent, there is no teaching of a process for forming a soluble reaction polyimide product in the proclaimed solvent systems, such as dimethylformamide (DMF) or N-methyl-2-pyrrolidone (NMP) nor of an anhydride activity above 0.17. Furthermore, it has been found that when any polyimide dianhydride is prepared using DMF as solvent, there is no detectable anhydride absorption peak (1840 cm.sup.-1) in the product's infrared spectrum whereas an identical preparation in phenol or m-cresol gives a product with a substantial absorption at 1840 cm.sup.-1. However separation of the phenol or cresol involves prolonged and expensive separation to recover the polyimide product so that it may be used in a solvent more suitable for lamination.
The reason for this failure to produce polyimide dianhydrides with high anhydride reactivity (I.sub.R) when these amide solvents are used is not known, but it is suspected that there is a reaction occurring between the anhydride groups and these solvents during imidization at high temperatures. As has been found, a complete imidization in these solvents usually requires temperatures above 160.degree. C. (but preferably below 170.degree. C.) for several hours. It is quite possible that the DMF, as well as related solvents containing N-alkylated amide groups, react with the anhydride groups. This results in intermediates which have low anhydride activity, defined herein as I.sub.R. However, whatever the reason, it has been found impossible to prepare well reacted polyimide-epoxy thermoset resins from a polyimide dianhydride which is prepared in DMF or a related solvent under known processes.
A problem encountered when an anhydride-terminated polyamic acid is used for preparing the polyimide-epoxy thermoset laminate is that delamination occurs when the molding or post-curing temperature is above 180.degree. C. However, since the use of high molding temperatures of this kind (above 180.degree. C.) is required to insure complete cyclization of imide groups and a complete curing of polyimide-epoxy thermoset resin, laminates produced from the polyamic type are unsatisfactory.
It is important therefore, that the method, including the solvent, used for preparing the polyimide dianhydride, is one that produces these intermediates with a high anhydride activity (I.sub.R) and good conversion to imide structures. The high I.sub.R, good solubility and low fusion temperatures are desirable for good subsequent reaction with a polyepoxide.
In this field of polyimides there are a number of terms which are commonly used, such as "degree of polymerization" (DP), "molar ratio of monomers" (r.sub.m), "statistical average of structure reoccurrence" (n), "degree of imidization" (C), "relative reactivity" (I.sub.R), and the "ratio of epoxy equivalents to anhydride equivalents." These are defined as follows:
The Molar Ratio of starting monomers is represented as r.sub.m or X/Y, with X representing moles of diamine and Y the moles of dianhydride.
Degree of Polymerization (DP)--Polyimides may be prepared by reacting X moles of diamine with Y moles of dianhydride. To produce an anhydride-terminated polyimide, Y is greater than X. The statistical average "degree of polymerization" (DP) may be calculated on the basis that the formation of the intermediate amic acid groups may be negated by the relatively long reaction periods used as compared to the relatively short time for amic acid formation. Therefore: EQU DP=(1+r.sub.m)/(1-r.sub.m)
Statistical Average of Structure Reoccurrence (n) is equal to: EQU (DP-1)/2=r.sub.m /(1-r.sub.m)
For example, where r.sub.m is 0.5 and DP is 3, then n is 1.
Degree of Imidization (C) is equal to the amount of water distilled from the reaction divided by the amount of water theoretically to be removed by complete imidization. This is equal to (2n.times.18) grams for making one gram mole of polyimide.
Relative Reactivity (I.sub.R) is the ratio of the intensity peak ratio of the absorption peak of the anhydride group at 1840 cm.sup.-1 to that of the imide group at 1790 cm in the Infrared Spectrum of the polyimide.
Equivalents Ratio of epoxy to anhydride (R) is:
R=(No. of equivalent weights of polyepoxide)/(No. of equivalent weights of dianhydride)
wherein the number of equivalent weights of a component is the weight of the component divided by the equivalent weight of the component.